GNSS (Global Navigation Satellite Systems) is a general term for positioning systems using satellite. The GNSS includes GPS (Global Positioning System), GLONASS (Global Navigation Satellite System) and GALILEO. Frequency bands of the respective satellite positioning systems are as follows.                GPS (L1) 1575.42±1.023 MHz        GALILEO (E1) 1575.42±2.046 MHz        GLONASS (L1) 1595.051 MHz to 1605.8865 MHz        
Therefore, in case that full bandwidth of the GPS, the GLONASS and the GALILEO is intended to be covered by one microstrip antenna, requirements of the antenna such as an axial ratio have to be satisfied in a wide frequency band (whose bandwidth is 32.5125 MHz) of 1573.374 MHz to 1605.8865 MHz. The frequency band of 1573.374 MHz to 1605.8865 MHz will be hereinafter also mentioned as “GNSS frequency band”.
FIG. 9 is a perspective view of a first configuration example of a one-point-feed microstrip antenna. In FIG. 9, a ceramic plate 13 (dielectric plate) is provided on a central portion of a substrate 14 serving as a ground conductor. A silver electrode 12 (patch electrode) serving as an antenna element is formed in the ceramic plate 13. Electric power is fed to the silver electrode 12 by one feed pin 11 that pierces through the substrate 14 and the ceramic plate 13. FIG. 10 is a frequency characteristic graph of an axial ratio in the case where the ceramic plate 13 measures 20 mm×20 mm×4 mm and the substrate 14 measures 60 mm×60 mm×0.8 mm in the microstrip antenna in FIG. 9. Although it is desirable that the axial ratio is normally equal to or lower than 4 dB, the axial ratio deteriorates up to 15 dB within the GNSS frequency band in the characteristics shown in FIG. 10.
FIG. 11 is a perspective view of a second configuration example of the one-point-feed micros trip antenna. The second configuration example shown in FIG. 11 is different from the first configuration example shown in FIG. 9 in the point that a large conductor ground plate is separately provided for antenna operation in addition to the substrate 14. In this case, the substrate 14 is released from the function as the ground for antenna operation, so that the substrate 14 can be miniaturized. As examples of the dimensions, the substrate 14 measures 21 mm×21 mm×0.8 mm and the conductor ground plate 15 measures 60 mm×60 mm. Also in the second configuration example, the frequency characteristics of the axial ratio are substantially the same as those in the first configuration example shown in FIG. 10.
FIG. 12 is a perspective view of a third configuration example of the one-point-feed microstrip antenna. The third configuration example shown in FIG. 12 is different from the first configuration example shown in FIG. 9 in the point that the size of the ceramic plate 13 and the size of the silver electrode 12 are increased. FIG. 13 is a frequency characteristic graph of an axial ratio in the case where the ceramic plate 13 measures 50 mm×50 mm×4 mm and the substrate 14 measures 60 mm×60 mm×0.8 mm in the microstrip antenna in FIG. 12. It will be understood by comparison between FIG. 10 and FIG. 13 that the axial ratio is improved due to the increase in size. However, even in the frequency characteristics of FIG. 13, the axial ratio deteriorates up to 4.5 dB within the GNSS frequency band so it cannot be said that the antenna has sufficient performance for use as a GNSS antenna.
In this manner, the one-point-feed microstrip antenna has a simple configuration but the size of the microstrip antenna increases inevitably when the operable frequency band is intended to be widened. It is difficult to obtain the required axial ratio (for example, which is equal to or lower than 4 dB) within the GNSS frequency band even if the size of the microstrip antenna is increased. As another method for widening the frequency band of a microstrip antenna, it is effective that two feed points are provided and electric signals whose phases are different by 90° from each other are fed to the two feed points.
FIG. 14 is a perspective view of a two-point-feed microstrip antenna. A ceramic plate 13 measures, for example, 20 mm×20 mm×4 mm. In the case of the two-point-feed system, a feeder circuit providing signals with a phase difference of 90° at the two feed pins 11 has to be mounted on a substrate 14.
FIG. 15 is a perspective view of the substrate 14 in which a hybrid circuit called branch-line coupler is provided as the feeder circuit. Each side of a conductor pattern 16 formed substantially into a square shape has a length of approximately λ/4. When the substrate 14 is, for example, a glass epoxy substrate, λ/4 corresponds to about 27 mm, taking into account the shortening effect due to a dielectric constant of the substrate 14. In addition, when an LNA (Low Noise Amplifier), a cable attachment pattern, etc. are also disposed to the substrate 14, the size of the substrate 14 is required to be equal to or larger than, for example, 40 mm×40 mm. Thus, the size of the substrate as a microstrip antenna becomes large.
FIG. 16 is a perspective view of a substrate 14 provided with a hybrid circuit, as a feeder circuit, in which a λ/4 transmission line is added to one output of a Wilkinson divider. A conductor pattern 17 is the Wilkinson divider and a conductor Pattern 18 is the λ/4 transmission line. In this case, the size of the substrate 14 is further larger than that in the case of the branch-line coupler. Thus, the size of the substrate as a microstrip antenna becomes large.